(a) Field of the Invention
The present invention relates to a protection circuit, a resonant converter including the same, and a protection method thereof. More particularly, the present invention relates to a method for protecting a resonant converter that has failed in zero voltage switching.
(b) Description of the Related Art
FIG. 1 schematically shows a conventional resonant converter.
As shown in FIG. 1, the resonant converter has two switches SW1 and SW2 that are alternately turned on/off. A resonance is generated between leakage inductance and magnetizing inductance of a transformer formed by a primary coil 1 and a secondary coil 2, and a capacitor 3, and a current Ip is generated by the resonance. A rectifying unit 4 generates a direct current DC by rectifying a current flowing through the secondary coil 2.
FIG. 2 shows a correlation between a gain that is a ratio (Vout/Vin) of an input voltage Vin and an output voltage Vout, and a switching frequency fs. The switching frequency implies an operation frequency that alternately turns on/off two switches SW1 and SW2.
As shown in FIG. 2, in the gain curve depending on a switching frequency, an operation region of the resonance converter can be divided into a zero voltage switching region and a zero current switching region with reference to a peak switching frequency ft (hereinafter referred to as a reference frequency). Here, a gain is maximized at the peak switching frequency. The right-side region (the case that the switching frequency fs is greater than the peak switching frequency ft) is a zero voltage switching region (hereinafter referred to as a ZVS region), and the left-side region (the case the switching frequency fs is smaller than the peak switching frequency ft) is a zero current switching region (hereinafter referred to as a ZCS region). The zero voltage switching implies that a voltage difference across switch is substantially zero before the switch turns on. The zero current switching implies that no current flows through a channel of the switch at a turn-off time of the switch. When the zero current switching occurs in the resonant converter using a MOSFET-type switch, it can be occurred reverse recovery at a body diode thereof and hard switching occurs at a turn-on time of the switch.
FIG. 3A shows a current flowing through a switch SW1 and a current Ip when a conventional resonant converter operates in the ZCS region.
As shown in FIG. 3A, when the converter operates in the ZCS region, reverse recovery occurs at a turn-off time of the switch SW1, and hard switching occurs at a turn-on time of the switch SW1.
FIG. 3B shows a current flowing through the switch SW1 and the current Ip when the conventional resonant converter operates in the ZVS region.
As shown in FIG. 3B, when the converter operates in the ZVS region, a current with a negative direction flows through the switch SW1 at the turn-on time of the switch SW1 so that a voltage difference across switch SW1 can be neglected.
As described, the reverse recovery at the body diode of the MOSFET switch and the hard switching occurring at the turn-on time of the switch can be prevented when the resonant converter operates in the ZVS region.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.